There's usually a bonus star promotion going on too, like an extra star or two on espresso drinks or a bunch for buying a drink, a breakfast sandwich and a baked good.
I always feel like a jerk though if there is a huge ass line and I make them ring me up 5 separate times lol (we usually buy two drinks plus 2-3 bakery items if we go there for breakfast). Sometimes, I'll just have them split it in two transactions, which gets me one extra star and not as much time.
I need 22 to make gold. What does gold get me, anyway?
Every so many stars, you get a free drink/item. I don't know how many stars it is--I have the app on my phone which tracks it, and I just check it every so often to see if I have a free item coming up. They also send me emails when I have a free item.
There may be other perks too, but that is the one I'm aware of/use the most.
I think the only limit on stars per day is that it only lets you add two star codes from packages you purchase. I hold my star codes until there's a promo of bonus stars for packaged coffee.